The majority of commercially available distillate fuels are derived from crude oil. Crude oil is in limited supply, and fuel derived from crude oil is often contaminated with sulfur and nitrogen compounds that contribute to acid rain. For these reasons, efforts have focused on methods for forming distillate fuels from feedstocks other than crude oil.
Several methods have been developed for converting natural gas to distillate fuels. One method involves converting methane to methanol, and then converting the methanol to higher molecular weight products. Another method involves converting methane to synthesis gas (“syngas”), a mixture of carbon monoxide and hydrogen gas, and subjecting the syngas to Fischer-Tropsch synthesis. In the Fischer-Tropsch synthesis process, liquid and gaseous hydrocarbons are formed by contacting syngas with a Fischer-Tropsch catalyst under suitable process conditions. Fischer-Tropsch synthesis processes produce a spectrum of hydrocarbons from methane, ethane and low molecular weight olefins to high molecular weight waxes, depending on the type of catalyst used and the process conditions selected, by way of example, the temperature, syngas ratio and other variables.
The low molecular weight olefins can be oligomerized, and the waxes can be hydrocracked, with the products optionally isomerized, to provide hydrocarbons useful in gasoline compositions. The feedstock to the hydrocracker may need to be hydrotreated to remove oxygenates. Both the hydrotreating and hydrocracking steps require the use of hydrogen and are performed at relatively high temperatures and pressures. Methane and ethane are low value products from the Fischer Tropsch process, and since they are feeds for the syngas generator, these products are typically recycled. Recycling of the methane and ethane may be difficult, capital intensive, and reduce the efficiency of the Fischer Tropsch conversion process. Therefore, it is undesirable to produce large amounts of methane in the Fischer-Tropsch reaction. Typically, yields of methane above 10% are economically unacceptable in a Fischer Tropsch process.
Fischer Tropsch processes and processes for upgrading hydrocarbon products are known in the art. By way of example, U.S. Pat. No. 4,502,945 ('945) to Olbrich et al. discloses a process for producing olefins from normal paraffins and slightly branched paraffins by contacting a feed of the paraffins with an intermediate pore size zeolite. U.S. Pat. No. 4,622,308 to Koideda et al. discloses a Fischer-Tropsch process for producing hydrocarbons from syngas using a catalyst that comprises a combination of an iron-containing Fischer-Tropsch catalyst, a zeolite, and at least one metal selected from the group consisting of ruthenium, rhodium, platinum, palladium, iridium, cobalt, and molybdenum. However, the catalyst of Koideda results in the formation of relatively high amounts of methane.
Accordingly, there exists a need for integrated Fischer Tropsch processes that produce relatively high amounts of high value products (i.e., hydrocarbons useful in gasoline compositions) and relatively low amounts of low value products (i.e., methane).